1. Field of the Invention
This invention relates to an internal combustion engine of the split type operable on less than all of its cylinders when the engine load is below a given value and, more particularly, to such an internal combustion engine equipped with a NOx-reduction structure.
2. Description of the Prior Art
In general, internal combustion engines demonstrate higher efficiency and thus higher fuel economy when running under higher load conditions. In view of this fact, split type internal combustion engines have already been proposed which include split engine control means for cutting off the supply of fuel to some of the cylinders so as to render them inactive when the engine load is below a given value. This creates a relative increase in the load on the remainder cylinders, resulting in higher fuel economy at low load conditions.
One difficulty with such split type internal combustion engine is that during a split engine mode of operation, the initial combustion temperature in the active cylinders is increased while the exhaust gas temperature is reduced as compared to the temperatures which would exist during a full engine mode of operation at the same conditions. This result in an increase in the formation of NOx during a split engine mode of operation.